Preheat the oven to 350°F (175°C).
In a mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the egg and vanilla extract, mixing until fully combined.
In a separate bowl, whisk together the flour and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Spoon the cookie dough into a greased muffin tin, filling each cup about two-thirds full.
Bake for 10-12 minutes, or until the edges are golden brown.
Allow the cookie cups to cool completely in the muffin tin before preparing the filling.
In a mixing bowl, combine mascarpone cheese, powdered sugar, and cooled coffee, whisk until smooth.
Pipe or spoon the mascarpone filling into the cooled cookie cups.
Dust with cocoa powder before serving.